fixes #350; fixed a problem with the additional tour guide widget
fixes #352; birthdate is not marked as  mandatory when editing tour
participations
fixes #349; tour guide report cancel reason is now deleted when the tour
has been carried out
Fixes #348 display difficulty abbreviations
fixes #348; tour abbreviations link is now shown once on the tour form and
tour filter
fixes #347; notification emails are no longer sent on tour participation
status updates after the tour has started
Merge tag 'v1.8.3' into integration

Tagging v1.8.3; some small bug fixes